1. Field of the Invention
The present invention relates to an optical member composed of a fluoride crystal as a raw material for producing an optical element for constructing an optical system of an optical instrument such as a camera, a microscope, a telescope as well as of a projection exposure apparatus for photolithography such as a stepper, and a method for evaluating the same. The present invention also relates to an optical system and a projection exposure apparatus incorporated with an optical element produced of the optical member.
2. Description of the Related Art
In recent years, the lithography technique for drawing an integrated circuit pattern on a wafer is rapidly advanced. The demand to increase the degree of integration of the integrated circuit is growing year after year. In order to realize the high degree of integration, it is necessary to enhance the resolving power of the projection optical system of the projection exposure apparatus. The resolving power of the projection optical system is determined by the wavelength of a light beam to be used and NA (numerical aperture) of the projection optical system. That is, the resolving power can be increased by further shortening the wavelength of the light beam to be used (realization of short wavelength) and/or further increasing NA of the projection optical system (realization of large diameter).
At first, a description will be made about the realization of the short wavelength of the light beam. The wavelength of the light source to be used for the projection exposure apparatus has been already changed to the g-ray (wavelength: 436 nm) and the i-ray (wavelength: 365 nm). It is investigated to use light beams having shorter wavelengths in the future, including, for example, the KrF excimer laser light beam (wavelength: 248 nm) and the ArF excimer laser light beam (wavelength: 193 nm). However, if commonly used multicomponent optical glass is used as a lens material for an image-forming optical system such as a projection optical system to be used for the light beam as described above, the transmittance is considerably lowered.
Therefore, silica glass or fluoride crystal, for example calcium fluoride crystal is generally used as an optical member for the optical system of the projection exposure apparatus which uses the excimer laser as the light source. In order to satisfy the image formation performance required for the optical member to be used for the optical system of the excimer laser projection exposure apparatus, it is desirable to use a single crystal in the case of a crystalline material.
As the performance of the projection exposure apparatus is highly enhanced, a calcium fluoride single crystal having a large diameter, i.e., a diameter of about xcfx86120 mm to xcfx86350 mm is recently required in order to increase NA. Such a calcium fluoride single crystal has a small refractive index and a small dispersion (dependency of the refractive index on the wavelength) as compared with the commonly used optical glass and the silica glass. Therefore, a merit is also obtained such that the chromatic aberration can be corrected by using the calcium fluoride single crystal together with the optical member composed of a material such as silica glass. It is also possible to obtain a single crystal having a large diameter exceeding xcfx86120 mm.
The calcium fluoride single crystal, which has the advantages as described above, has been hitherto used as lens materials for cameras, microscopes, and telescopes other than as the optical material for the projection exposure apparatus. Recently, single crystals of barium fluoride and strontium fluoride, which are fluoride single crystals other than the calcium fluoride single crystal, attract the attention as optical materials for the next generation, because they belong to the same cubic system and they have similar properties.
A variety of single crystal growth methods are known as the method for producing the fluoride single crystal, including, for example, the melt method such as the Tammann method and the Bridgman method (also referred to as the Stockbarger method or the pull-down method). A method for producing the calcium fluoride single crystal based on the Bridgman method will be described below by way of example. FIG. 2 conceptually shows a growth apparatus for the calcium fluoride single crystal based on the Bridgman method.
In order to produce the calcium fluoride single crystal for the purpose of the use in the ultraviolet or vacuum ultraviolet region, a calcium fluoride raw material having high purity, which is produced by means of chemical synthesis, is generally used as the raw material. If any powder is used as the raw material to grow the crystal, the volume is greatly decreased when the raw material is melted. In order to avoid such an inconvenience, the crystal is generally grown by using a raw material obtained by semi-melting the powder once or obtained by crushing the product obtained by semi-melting the powder once.
At first, a crucible, which is filled with a semi-molten material or a crushed material thereof, is set in the growth apparatus. The interior of the growth apparatus is maintained in a vacuum atmosphere of 10xe2x88x923 to 10xe2x88x924 Pa. Subsequently, the temperature in the growth apparatus is raised to a temperature which is not less than the melting point of calcium fluoride (1370xc2x0 C. to 1450xc2x0 C.) to melt the raw material.
At the crystal growth (grain growth) stage, the crucible is moved downwardly at a speed of about 0.1 to 5 mm/h, and thus the crystal growth is gradually advanced from the lower portion of the crucible. The crystal growth comes to an end when the uppermost portion of the melt is crystallized. The grown crystal (ingot) is gradually cooled to a temperature in the vicinity of the room temperature so that the crystal (ingot) is not broken. After that, the interior of the growth apparatus is open to the atmospheric air, and the ingot is taken out.
A crucible made of graphite is generally used for the crystal growth. The crucible is pencil-shaped with its tip having a conical configuration. Therefore, the crystal growth is started from the tip having the conical configuration disposed at the bottom of the crucible. The crystallization is gradually advanced, and the ingot is finally obtained.
A seed crystal is sometimes introduced into the tip portion in order to control the crystal plane orientation of the ingot. However, in general, when a large fluoride crystal is produced by means of the Bridgman method, it is considered that the crystal growth orientation does not obey any law, and the crystal direction of the ingot is randomly determined every time when the crystal growth is performed. Especially, in the case of a large ingot having a diameter exceeding xcfx86120 mm, it is extremely difficult to control the crystal plane orientation.
A large residual stress exists in the ingot taken out from the crucible after the crystal growth. Therefore, a simple heat treatment is performed while retaining the ingot shape as it is. The ingot of the calcium fluoride single crystal obtained as described above is cut and processed into an appropriate size depending on an objective product.
When an optical element, in which the crystal plane orientation causes no problem, is produced, the ingot is cut horizontally to have a parallel plate-shaped configuration (cut into round slices) in order to cut out raw materials from the ingot more efficiently. A heat treatment is applied to the cut raw materials in order to obtain desired image formation performance (uniformity of refractive index and reduction of stress induced birefringence).
When an optical element, in which the crystal plane orientation should be considered, is produced, for example, when the optical axis is made perpendicular to the {111} crystal plane, then the {111} crystal plane of the fluoride single crystal ingot is measured. The raw material is cut out so that the {111} plane resides in the two parallel planes, and then a heat treatment is performed.
It has been hitherto considered that the single crystal based on the cubic system has no intrinsic birefringence, or if any, the single crystal based on the cubic system has an intrinsic birefringence at a level at which no problem occurs. Therefore, it has been considered that the birefringence causes no serious problem in the optical design, for example, in the case of the single crystal of calcium fluoride provided that the birefringence, which is caused by the thermal stress in the production steps, is reduced to a level at which no problem occurs.
However, in recent years, the following fact has been revealed. That is, the intrinsic birefringence tends to depend on the wavelength. When the wavelength of the light beam to be used is longer than about 250 nm, then the intrinsic birefringence is small, and it can be almost neglected. On the other hand, the shorter the wavelength is, the larger the intrinsic birefringence is. For example, in the case of the {110} plane, it has been revealed that the values of the birefringence are not more than 0.2 nm/cm, 3.4 nm/cm, and 11.2 nm/cm for the light beams having wavelengths of 633 nm, 193 nm, and 157 nm respectively. The wavelength of 157 nm is the wavelength of the F2 laser beam. It has been revealed that the influence of the intrinsic birefringence, which is exerted on the image formation performance of the optical system, cannot be neglected in relation to the light beam having a short wavelength as described above.
The way of distribution of the intrinsic birefringence in the optical element depends on the relationship between the optical element and the direction of the crystal. Therefore, the distribution also changes in accordance with the change of the relationship between the crystal plane orientation of the single crystal and the direction of the optical axis when the single crystal is processed and machined into an optical member. This fact will be briefly explained. FIG. 3 schematically shows the crystal plane orientations of the fluoride crystal based on the cubic system. As shown in FIG. 3, no birefringence exists in the directions of the [100] axis and the [111] axis. On the other hand, the birefringence is maximum in the direction of the [110] axis. The crystal plane orientations are represented in accordance with the Miller indexes. The Miller index is the inverse number of the value obtained by dividing the distance from the point of intersection between the crystal plane and each crystal axis to the lattice origin of the crystal by the lattice spacing of each crystal axis. For example, in the case of the cubic system as in calcium fluoride, all of the lattice spacings of the respective crystal axes are identical. Therefore, it is assumed that the lattice spacing is xe2x80x9caxe2x80x9d. On this assumption, when a certain crystal plane intersects the crystal axes at points separated from the lattice origin by a/h, a/k, and a/l respectively, the crystal plane orientation is represented as (hkl) with the Millar index. In this expression, h, k, and l are integers. According to the Millar index, the direction [hkl] is perpendicular to the crystal plane (hkl) in the cubic system. The directions, which are in a symmetric relation, are represented by one index to make representation with parentheses  less than   greater than . The crystal planes, which are in a symmetric relation, are also represented by one index to make representation with parentheses { }. For example, all of the diagonal lines of the cube [111], [1-11], [-1-11], [-111] are represented by  less than 111 greater than . The crystal planes of the cube (100), (010), (-100), (0-10), (001), (00-1) are represented by {100}.
Therefore, for example, in the case of an optical system in which the  less than 111 greater than  axis is coincident with the optical axis, the peaks of the birefringence distribution (birefringence symmetry axes) exist in the three directions as shown in FIG. 4. When the  less than 100 greater than  axis is the optical axis, the birefringence symmetry axes exist in the four directions as shown in FIG. 5. When the  less than 110 greater than  axis is the optical axis, the birefringence symmetry axes also exist in the four directions as shown in FIG. 6.
As explained above, it is appreciated that the intrinsic birefringence in the optical element is distributed depending on the crystalline orientation. In view of the above, a method has been suggested, in which the peaks of the birefringence distribution are counteracted with each other by combining a plurality of optical elements while rotating them so that they mutually have predetermined angles about the center of the optical axis. This is referred to as xe2x80x9cclockingxe2x80x9d.
For example, FIG. 7 shows a state in which two optical members are arranged while mutually rotating the  less than 110 greater than  axis by 60xc2x0 about the center of the optical axis in an optical system in which the  less than 111 greater than  axis is the optical axis. On the other hand, FIG. 8 shows a state in which two optical members are arranged while mutually rotating the  less than 110 greater than  axis by 45xc2x0 about the center of the optical axis in an optical system in which the  less than 100 greater than  axis is the optical axis. Further, FIG. 9 shows a state in which four optical members are arranged while rotating the  less than 111 greater than  axis and the  less than 100 greater than  axis by 45xc2x0, 90xc2x0, and 135xc2x0 respectively about the center of the optical axis in an optical system in which the  less than 110 greater than  axis is the optical axis.
As described above, the influences on the image formation performance, which would be caused by the birefringence distributions of the respective optical elements, can be counteracted with each other by setting the plurality of optical members so that they mutually form predetermined angles (by performing the clocking). Accordingly, the point image intensity distribution (Strehl Intensity) is improved, and it is possible to avoid the deterioration of the image formation performance.
However, even when the clocking is performed, the image formation performance is not improved yet in some cases.
The present invention has been made in order to solve the problems involved in the conventional technique, an object of which is to provide an optical element composed of a fluoride crystal having satisfactory image formation performance. A second object of the present invention is to provide an optical element in which the occurrence of birefringence is sufficiently suppressed even in the case of a light beam having a short wavelength of not more than 250 nm, a projection optical system provided with the same, and a projection exposure apparatus provided with the same.
As a result of studies and investigations performed by the present inventors in order to solve the problems involved in the conventional technique, it has been found out that a region called twin exists in an optical element such as a lens obtained by processing a fluoride single crystal, and it has been ascertained that the twin generates a birefringence distribution and the image formation characteristics of the optical element are deteriorated thereby.
In general, the twin is formed such that two crystals, which have an identical chemical composition and an identical crystalline structure, are joined to one another in a relationship in which the two crystals are symmetrical with respect to a specified plane or axis. The two crystals as described above are related, for example, to have such crystal plane orientations that the  less than 111 greater than  axes are in a mutually identical direction but the  less than 100 greater than  are mutually inverted by 180 degrees. However, in this specification, for the convenience of explanation, the following terms are used for one in which the two crystals having the relationship as described above are joined to one another. That is, the crystal, which occupies the smaller region, is referred to as xe2x80x9ctwinxe2x80x9d, and the crystal portion, which occupies the smaller region, is referred to as xe2x80x9ctwin regionxe2x80x9d. According to studies performed by the present inventors, the following fact has been revealed. That is, the distribution of birefringence differs between the twin portion and the other portion in an optical element in which the twin exists therein. Therefore, even when the clocking is performed, it is impossible to sufficiently reduce the polarity of the birefringence distribution.
According to a first aspect of the present invention, there is provided a method for evaluating an optical member for photolithography composed of a fluoride crystal, characterized by comprising the steps of:
measuring a crystal plane orientation of the optical member; and
specifying a twin region on the basis of a result of the measurement. The present inventors have successfully obtained an optical element in which the birefringence distribution is suppressed, by specifying the twin region in the optical member on the basis of the knowledge described above, selecting only the optical member in which the twin region is not more than a predetermined ratio, and performing processing into the optical element by using the optical member. The evaluation method of the present invention may further comprise a step of calculating a ratio occupied by the specified twin region with respect to an effective region of the optical element to be produced from the optical member.
In the method for evaluating the optical member according to the present invention, the step of measuring the crystal plane orientation of the optical member may be performed by radiating an X-ray onto the optical member. In this procedure, the X-ray may be radiated onto the optical member on the basis of a Laue method, especially on the basis of the Laue method of a side surface reflection type.
The twin region may be specified as a total area of a region obtained by projecting the twin region in the effective region of the optical element to be produced from the optical member onto a plane perpendicular to an optical axis of the optical element. In this procedure, the effective region of the optical element is indicative of the region of light irradiation with the light beam which may be radiated onto the optical element. The effective region of the optical element may be an effective diametral area of the optical element or a partial diametral area of the optical element.
In order to evaluate the optical member, the method of the present invention may further comprise a step of judging that the optical member is usable when the total area is not more than 10% of the effective diametral area or the partial diametral area of the optical element.
According to a second aspect of the present invention, there is provided an optical element for photolithography to be used in a wavelength band in which a wavelength is not more than 250 nm, wherein:
a region, which is obtained by projecting a twin region of the optical element onto a plane perpendicular to an optical axis of the optical element, has a total area which is not more than 10% of an effective diametral area of the optical element. The optical element of the present invention has satisfactory image formation characteristics, because the optical element involves a small amount of the twin region which would cause the occurrence of the birefringence.
According to a third aspect of the present invention, there is provided an optical element for photolithography to be used in a wavelength band in which a wavelength is not more than 250 nm, wherein:
a region, which is obtained by projecting a twin region of the optical element onto a plane perpendicular to an optical axis of the optical element, has a total area which is not more than 10% of a partial diametral area of the optical element. The optical element of the present invention has satisfactory image formation characteristics, because the optical element involves a small amount of the twin region which would cause the occurrence of the birefringence.
According to a fourth aspect of the present invention, there is provided an optical system for photolithography to be used in a specified wavelength band in which a wavelength is not more than 250 nm, the optical system comprising:
the optical element according to the second or third aspect of the present invention; and
a lens barrel into which the optical element is incorporated. The optical system for photolithography may be a projection optical system.
According to a fifth aspect of the present invention, there is provided an exposure apparatus for photolithography to be used in a specified wavelength band in which a wavelength is not more than 250 nm, wherein:
the optical system according to the fourth aspect of the present invention is incorporated.
According to a sixth aspect of the present invention, there is provided a method for producing an optical system for photolithography to be used in a specified wavelength band in which a wavelength is not more than 250 nm, characterized by comprising the steps of:
preparing an optical member which is judged to be usable by using the evaluation method of the present invention;
grinding and polishing the optical member and coating the optical member with an antireflection film or a reflection film to produce an optical element having a predetermined shape; and
incorporating the optical element into a lens barrel to produce the optical system thereby.
According to a seventh aspect of the present invention, there is provided a method for producing an exposure apparatus for photolithography to be used in a specified wavelength band in which a wavelength is not more than 250 nm, characterized by comprising the steps of:
preparing an optical member which is judged to be usable by using the evaluation method of the present invention;
grinding and polishing the optical member and coating the optical member with an antireflection film or a reflection film to produce an optical element having a predetermined shape;
incorporating the optical element into a lens barrel to produce an optical system thereby; and
attaching the optical system to a predetermined position of the exposure apparatus.